1. Field of the Invention
The present invention relates in general to improved brake assemblies for railway freight car trucks. More specifically, but without restriction to the particular use which is shown and described, this invention relates to the improved mounting of disc brake assemblies to the freight car truck.
2. Description of the Related Art
It is well known in the art to use disc brake assemblies as the braking mechanism for railway trucks, including passenger and freight car trucks. It is also known in the art that the disc brake assemblies include brake discs, disc brake calipers, and brake pads. In use, the brake discs are typically mounted on the truck wheel axle assembly, either directly to the axle or through the wheel. Under either mounting, the brake discs are unsprung. That is, the brake discs do not move relative to the wheel axle assembly.
The disc brake calipers are conventionally mounted on the truck frame and releasably engage the brake discs. Attached to the disc brake calipers are the brake pads which, in operation, contact the brake discs to provide a braking force to the wheels. Passenger and freight car trucks feature a primary suspension between the wheel axle assembly and the truck frame that allows relative vertical motion between the wheel axle assembly and the truck frame and accompanying disc brake calipers. Thus, the disc brake calipers are sprung relative to the wheel axle assembly and brake discs.
On passenger car trucks, this relative motion between the sprung brake calipers and unsprung brake discs has been limited by several design factors: loaded tare weight ratio is small on passenger trucks to minimize deflections due to static load change; the disc brake calipers conventionally have been located longitudinally inboard of the axle thereby limiting the deflection due to truck frame pitching; and a secondary suspension has been located between the truck frame and bolster to absorb the majority of vertical suspension deflection, thereby allowing the primary suspension to be stiffer. Taken together, these passenger car truck design factors have limited primary spring deflection sufficiently to accommodate sprung brake calipers and unsprung brake discs and has not resulted in the problems encountered by freight car trucks, discussed below.
As stated above, when a freight truck frame is mounted on a primary suspension, such as a primary spring, the disc brake caliper is sprung relative to the brake discs. During freight car operation, the sprung mounting of the caliper has resulted in an undesirable amount of relative vertical motion between the unsprung brake disc and the sprung brake caliper due to various conditions, such as, freight load, dynamic bounce and truck frame pitching. This undesirable relative vertical motion is amplified during high-speed operation of freight cars, which use softer primary springs to ensure safe operation and acceptable ride quality over existing track. The use of soft springs creates large spring deflections, which has been shown to create two common problems. First, the brake pads, which are part of the caliper assembly, can edge partially off of the disc, thereby reducing braking efficiency. Second, the brake pad reaction forces are transmitted through the brake caliper assembly to the truck frame. These reaction forces result in a pitching moment in the truck frame, which can unload individual primary springs. When the truck frame pitching is severe, individual springs may completely unload creating the possibility of unseating of the springs, or even losing springs from the truck assembly.
The present invention overcomes the aforementioned problems of excessive motion between the brake disc and caliper, and potential spring unloading.
The present invention is directed at eliminating the known problems associated with relative motion between the brake disc and caliper due to load conditions and other dynamic forces which develop during high speed freight car operation. The invention achieves these objectives by providing a disc brake assembly wherein the brake caliper is mounted to the equalizer beam, which is unsprung relative to the wheel axle assembly. With this construction, both the brake disc and brake caliper are unsprung members, and unsprung relative to each other. Because these members are unsprung, the relative displacement between the brake disc and caliper due to load variation, dynamic bounce, and braking-induced pitching is essentially eliminated. Moreover, the possibility of primary spring unloading is also eliminated.